There are a couple of instances in the bible that show people “looking back”, and having to pay the consequence for it.
The first example is with Lot's wife who, as she and her family fled their wicked city of Sodom. She looked back once as they ran away and was turned into a pillar of salt.
"And it came to pass, when they had brought them forth abroad, that he said, Escape for thy life; look not behind thee, neither stay thou in all the plain; escape to the mountain, lest thou be consumed." (Gen 19:17)
"Then the LORD rained upon Sodom and upon Gomorrah brimstone and fire from the LORD out of heaven; And he overthrew those cities, and all the plain, and all the inhabitants of the cities, and that which grew upon the ground. But his wife looked back from behind him, and she became a pillar of salt." (Gen 19:24-26)
Why would God strike her down over a mere glance? Wouldn't you be curious if you were her? I'd be curious to look back and see God destroying my old city with fireballs! But His command was not to look back… she did that and so she was killed.
Another example of someone who “looked back” is a man whose father had just died (Matthew 8:20-21).
Jesus said, "come follow me", but this fellows father had just died so he said, "Let me go bury my father first. I want to pay my respects, I want to grieve. I need time to mourn." But our Lord stuns us with the statement: "Let the dead bury their own dead." In other words, let the spiritually dead bury the physically dead. Leave all that behind.
And so this man who had just lost his father might have missed out on Jesus because he wanted to grieve and mourn. Does this seem like the merciful, gracious Lord that we know? How can this be? There’s an important reason for that.
God had a lesson for us in these examples. The lesson is this: When you come to Jesus, you must repent. Repentance means not only can you not go back to your old ways, but you must not even look back! Look back at what? When the bible shows these examples it means that we can't look back at our sin. We’ve got to have no intention of turning back, or even looking back at our sin.
Suppose there’s some sin in your life that you cannot give up. Don’t fool yourself thinking you can come to the Lord like that! These kinds of people want Heaven without Christ; eternal life without righteousness; a resurrection without a cross first. On the other hand, I know people who say they don’t follow God simply because they have too much sin in their life that they don’t want to give up. At least these people are honest! These people are much more likely to enter the kingdom of God than the people who call themselves Christians but have zero intention of forsaking their sins!
God is absolutely firm when it comes to our sin and repentance. When we come to Him, we cannot follow Him while still looking backward. If you’re not looking back you’re looking forward – looking only to Jesus. Jesus commands that we pick up our cross and follow Him. A cross is large and heavy. You can’t also be dragging along your bag of sins along with it. You must look straight forward at the Lord and follow Him along the narrow path without looking back, having zero intention of ever returning to your old ways.
"Behold, you are made whole. Sin no more lest a worse thing come to you." (Joh 5:14)